Powderless etching



United States Patent 3,239,466 POWDERLESS ETCIHNG Clayton W. Hoornstra, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed June 10, 1963, Ser. No. 286,512 3 Claims. ((11. 252-794) This invention relates to an improved etching bath for powderless etching and the method of etching printing plates, nameplates, templates and the like with the improved bath. The improved bath is especially adapted to the etching of magnesium-base alloys containing at least 1 percent by Weight of aluminum.

In the conventional method of making a metal printing surface, such as a photoengraving, a plate of an acidsoluble metal such as magnesium and its alloys, is coated with a light-sensitive coating or enamel which is applied to the metal surface which is usually flat or cylindrical. The coated surface is exposed to light through a negative having an image thereon so as to produce a corresponding image in the coating. The exposed coating surface is developed forming an acid-resistant coating in the form of the image produced by the exposure. This acid-resistant coating is further hardened by heating and the image-bearing surface of the plate is then subjected to etching by an acid. As the etching proceeds, the acid tends to etch laterally and undercut the resist and thus distort the image. The standard method of preventing or reducing this undercutting at one time was to powder the plate, a time-consuming and difiicult operation. This method has now been replaced by the use of so-called powderless etching baths.

Heretofore, the powderless etching baths have employed nitric acid almost exclusively as the etching agent. The baths have also employed a filming agent and a water-immiscible liquid in addition to water. Many baths have also employed additional constituents in order to make the bath suitable for combination work, that is,

the etching of both line and half-tone areas in a photoengraving plate. The filming agent employed in the prior art, in powderless etching baths, to reduce lateral etching or undercutting of the resist has generally consisted of an anionic surface active material such as an alkylaryl sulfonate.

It has now been discovered that very simple powderless etching baths may be made up using pyridine in place of the filming agent of the art. If desired, the organic, water immiscible liquid may be omitted. Furthermore, there is the added flexibility that hydrochloric acid or sulfuric acid may be employed instead of nitric acid as the etching agent. This bath is suitably employed in combination work.

The metal plates etched using the bath and method of the present invention are desirably magnesium-base alloys containing at least 1 percent of aluminum. Magnesium-base alloys are those defined as containing at least 70 percent by weight of magnesium. An example of a suitable photoen-graving alloy consists of 3 percent by Weight of aluminum, 1 percent by weight of zinc and the balance commercial magnesium.

Etching agents which may be used in the present bath are the strong mineral acids, nitric acid, sulfuric acid and hydrochloric acid. The concentration of acid employed in the bath is best defined in terms of gram equivalents of which 0.5 to 4 should be employed per liter of bath, and more preferably, 1 to 2 gram equivalents of acid per liter of bath.

From 0.5 to 15 percent by weight of pyridine based on the total weight of the bath provides suitable protective action to minimize lateral etching and undercutting. A more preferred range is from 2 to percent by weight of pyridine.

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While an etching bath consisting very simply of an acid etchant, pyridine and water may be employed, it is often desirable to enhance the effectiveness of the bath by adding thereto an organic, water-immiscible liquid. This liquid may be a single compound or a blend of such compounds. The required properties of the liquid are that it be substantially stable in the presence of acid etchant at bath temperatures and that it have some solvency for the pyridine in the acidic bath medium. Substantially stable is defined as the non-occurrence of deterioration within a reasonable time period due to the presence of dilute acid etching agent which significantly alters the function of the organic, water-immiscible liquid additive within the bath in an unfavorable manner. Some solvency is defined as the ability to dissolve at least 0.01 percent by weight of pyridine in the organic, water-immiscible liquid portion of the bath.

Also, it is essential that this additive is a liquid at bath temperatures. Suitable organic materials which can be used individually or in combination are hydrocarbon aromatics, aliphatics and naphthenics boiling at from to 390 C. as, for example, ligroin, kerosene, gas oils, diethyl benzenes, tetramethyl benzenes, diisopropyl benzenes and dodecyl benzenes. Other examples of water-immiscible liquids includes turpentine, monochloroethyl benzene, ethylbutyl ketone, isophorone, methylhexyl ketone, d-limonen, diisodecyl phthalate, hexylacetate and the like. Generally, it may be said that waterimmiscible esters, ketones, terpenes, ethers and aliphatic naphthenics and aromatic hydrocarbons are operable.

Certain commercial solvents also serve very effectively as this additive to the bath. An example is a commercial aromatic solvent sold under the tradename of Solvesso 150, which is a mixture of approximately 90 percent alkyl benzenes, 2 percent naphthalene and 8 percent naphthenes. It has a flash point of 150 R, an aniline point of -l8 F., and a distillation temperature range at 760 mm. Hg as follows: initial boiling point of 303 F., 50 percent distilled at 378 F. and dry point at 415 F. Another commercial aromatic solvent employed has the tradename Penola H.A.N., and contains 84 percent aromatics, has a flash point of R, an aniline point of 2 F. and a distillation temperature range at 760 mm. Hg: initial boiling point of 340 F., 50 percent distilled at 446 F. and dry point at 532 F.

Up to 8 percent by weight of organic water-immiscible liquid may be employed if desired. A particularly favorable proportion of organic, water-immiscible liquid to employ is from 8 to 30 percent by weight based on the quantity of pyridine employed, and even more desirable, from 10 to 18 percent by weight based on the amount of pyridine in the bath.

In the practice of the present invention, it has been found desirable to impinge the bath upon the surface to be etched as by spraying, but much more preferable, by splashing. In the formulation of the etching bath, it has been found to be a general rule that as the conc ntration of the acid etching bath is increased within the aforementioned limits, it is desirable to also increase the quantity of pyridine employed. Due to bath depletion as the etching process is carried out, it is simpler and more convenient to designate amounts of components in terms of their initial concentrations.

The average bath temperature may be within the range of from- 60 to F. but a preferred range is from 70 to 90 F.

To carry out the etching process, it is preferred to employ an etching machine of the type disclosed in US. Patent 2,669,048, issued February 16, 1954. In this etching machine, rotating, elongated paddles intermittently splash the etching bath upwardly against the image- -bearing surface of the object being etched. The splashing action of the paddles also serves to keep the bath in a homogeneous state. In a machine of the type described in the said patent, paddle speed may be varied from 400 to 1000 r.p.m., but preferably, is maintained in the range of 500 to 800 r.p.m.

A bath of this invention applied in the described manner for about 6 to 8 minutes will usually produce depths of etch from about 0.015 to 0.020 inch in line areas on the plate while producing proper depths in various other parts of the plate. Also, etch factors in line image areas from 15 to 30 are readily obtainable. Etch factor as used in this specification is defined as the ratio of the depth of the etch adjacent to a line of resist divided by one-half of the total loss in width of metal immediately beneath the resist. It is obviously desirable to have as high etch factors as possible for faithful reproduction of the image in relief. However, it should be noted that the etch factor can be sensitive to changes in depth. Thus, it should be considered as only one criterion of etching bath quality. The etching bath' of the invention has very good chemical stability as a function of time and is capable of producing uniform and good results on sensitive plates over a period of many hours.

The following examples are intended to be merely illustrative of the invention and should not be construed as limiting the scope of the invention.

EXAMPLE 1 A plate 2" X 3 /2" x 0.064" formed of a magnesiumbase alloy having a nominal composition of about 3- percent aluminum, 1 percent zinc and the balance commercial magnesium was coated with a conventional polyvinyl alcohol photoresist and exposed to a photographic negative having images including a 65 screen half-tone area with some highlight dropout areas and line areas comprising type lines and illustrations. After the exposure, the plate was developed leaving a resist corresponding to the image of the printed matter and the half-tone area, the remainder of the plate being bare. The resist image on the plate was set by heating the plate in an oven for 6 minutes at a temperature of about 450 F. The plate was then descummed by scrubbing it with a dilute, aque-, ous solution of nitric acid containing about 10 percent of 42 Baum nitric acid until the plate was bright. An etching bath was made up consisting of 640 ml. of nitric acid (specific gravity 1.42), and 4360 ml. of water. This bath was placed in a miniature etching machine of the type disclosed in Patent No. 2,669,048 and maintained at a temperature of 72 F. The plate was held with the image bearing side down at a height of about 1 inch above the uppermost point of travel of the 1%" diameter paddles. These paddles were positioned in the bath and splashed the bath upwardly against the image-bearing side of the plate. The paddles were rotated for a. period of about 1 minute at 600 r.p.m. The line areas were etched to a depth of 0.0023 inch and exhibited an etch factor of 3.

Another photoengraving plate prepared and descummed as described above, was placed in the etching machine after adding 510 ml. of pyridine to the bath. The bath was mixed briefly by rotation of the paddles and paddle speed was then adjusted to 55 r.p.m. Etching of this second plate was carried out for about 4 minutes. The etched plate was rinsed and dried and examined, and the line areas were found to be etched to a depth of 0.011 inch and exhibited an etch factor generally greater than 9.

60 ml. of Solvesso 150 liquid was added to the bath and a third photoengraving plate prepared and descummed as described above was placed in the etching machine. The paddle speed was adjusted to 550 r.p.m. and etching was carried out for about 8 minutes. The etched plate was rinsed and dried and examined. The line areas were found to be etched to a depth of 0.018 inch and exhibited an etch factor of 30.

. etching was carried out for about 8 minutes.

EXAMPLE 2 An initial etching bath was made up consisting of 380 ml. of 95.5 percent by weight -HSO and 5620' ml. of water. This bath formula-tion was placed in an etching machine corresponding to that disclosed in PatentNo. 2,669,048 and maintained at a temperature of 72 F. A

photoengraving plate was developed and prepared and descummed as described in Example .1 and placed in the etching machine The paddle speed was adjusted to 600 r.p.m. and etching was carried out for only 30 seconds. This comparison plate was rinsed and dried and examined. The line areas were found to be etched to a depth of 0.006 inch and exhibited an etch factor of only 2.

240 ml. of pyridine were then added to the bath and mixed therewith. A second photoengraving plate prepared and des-cummed as described in'Example 1 was placed in the etching machine. The paddle speed was adjusted to 600 r.p.m. and etching was carried out for a period of 2 minutes. The etched, plate was rinsed and dried and examined. In the line areas, the plate had been etched to a depth of 0.0065 inch and showed an etch factor of 4.3.

Additions of an organic, water-immiscible liquid to the etching bath containing sulfuri acid and pyridine further improve the etch factor.

EXAMPLE 3 An initial bath consisting of 950 ml. of concentrated (37.5 percent by weight) hydrochloric acid and 4050 ml.- of water was placed in an etching machine corresponding to that disclosed in PatentNo. 2,669,048 and maintained at a temperature of 72. F. A photoengravingplate prepared and descummed as. described in Example 1 was placed in the etching machine. The paddle speed Was adjusted to 500 r.p.m. and etching was carried out for 1 minute. The etching plate was rinsed and dried and examined. In the line areas, it was found that the plate Was etched to a depth of 0.012 inch and. exhibited an etch factor of only 1.5.

Then ml. of pyridine and 30 ml. of Solvesso liquid were added to the bath in the machine and mixed therewith. A second photoengraving plate prepared and descummed as described in Example ,1 was placed inthe etching machine. The paddle speed was adjusted to 500 r.p.m. and etching was carried out for 1% minutes. The

etched plate was rinsed and dried andexamined. In the. line areas the plate had been etched to a depth of 0.0048

inch and exhibited an etch factor of 3.2.

An additional 390ml. of pyridine and 60 ml. of SOL vesso 150 liquid were added to the bath formulationgin the etching machine and admixed with the bath. A third photoengraving plate prepared as described in Example 1 was placed in the etching machine. Paddle speed was adjusted to 700 r.p.m. and etching was carried out over a period of 4.5 minutes. The etched plate was rinsed and dried and examined. In the line areas the'plate had been etched to a depth of 0.007 inch and exhibited an etch factor of 3.5. The half-tone. areas were also clearly developed.

Among the advantages of the present invention are .that it may be employed for the development of combination plates, that it provides for a simpler bath with fewer components, and that the bathis'eifective over a broader concentration range of components thusavoiding he usnalclose control heretoforerequired for combination baths,

Various modifications may be made in the etching bath and method of applying such bath without departing from the spirit of the invention, and it is to be understood that the scope of the invention is limited only by the appended claims as read in the light of the specification.

I claim:

1. A method of etching the surface of a magnesiumbase alloy containing at least 1 percent of aluminum and having an acid-resistant, partial coating thereon which comprises impinging an etching bath upon the metal surface to be etched, such bath consisting essentially of from 0.5 to about 4 gram equivalents per liter of an acid selected from the group consisting of nitric acid, sulfuric acid and hydrochloric acid; from 0.5 to 15 percent by weight of pyridine; up to 8 percent by weight of an organic, Water-immiscible liquid, said organic, water-immiscible liquid being substantially stable in the presence of the acid in the bath; and the balance water.

2. A method of etching the surface of a magnesiumbase alloy containing at least 1 percent of aluminum and having an acid-resistant, partial coating thereon which comprises impinging an etching bath upon the metal surface to be etched, such bath consisting essentially of from 0.5 to about 4 gram equivalents per liter of an acid selected from the group consisting of nitric acid, sulfuric acid and hydrochloric acid; from 2 to 10 percent by weight of pyridine; up to 8 percent by weight of an organic, Water-immiscible liquid, said organic, water-immiscible liquid being substantially stable in the presence of the acid in the bath; and the balance Water.

3. A method of etching the surface of a magnesiumbase alloy containing at least 1 percent of aluminum and having an acid-resistant, partial coating thereon which comprises impinging an etching bath upon the metal surface to be etched, such bath consisting essentially of from 1 to 2 gram equivalents per liter of an acid selected from the group consisting of nitric acid, sulfuric acid and hydrochloric acid; from 2 to 10 percent by Weight of pyridine; from 8 to percent by weight of an organic, waterimmiscible liquid based on the Weight of pyridine in the bath, said organic, water-immiscible liquid being substantially stable in the presence of the acid in the bath; and the balance Water.

References Cited by the Examiner UNITED STATES PATENTS 1,918,545 7/1955 Hoy 25279.2 XR 2,940,836 6/1960 Fishaber 25279.4 XR 2,965,577 12/1960 Heimann et a1. 252148 3,074,836 1/1963 Sherer 25279.4 XR

JULIUS GREENWALD, Primary Examiner. 

1. A METHOD OF ETCHING THE SURFACE OF A MAGNESIUMBASE ALLOY CONTAINING AT LEAST 1 PERCENT OF ALUMINUM AND HAVING AN ACID-RESISTANT, PARTIAL COATING THEREON WHICH COMPRISES IMPINGING AN ETCHING BATH UPON THE METAL SURFACE TO BE ETCHED, SUCH BATH CONSISTING ESSENTIALLY OF FROM 0.5 TO ABOUT 4 GRAM EQUIVALENTS PER LITER OF AN ACID SELECTED FROM THE GROUP CONSISTING OF NITRIC ACID, SULFURIC ACID AND HYDROCHLORIC ACID; FROM 0.5 TO 15 PERCENT BY WEIGHT OF PYRIDINE; UP TO 8 PERCENT BY WEIGHT OF AN ORGANIC, WATER-IMMISCIBLE LIQUID, SAID ORGANIC, WATER-IMMISCIBLE LIQUID BEING SUBSTANTIALLY STABLE IN THE PRESENCE OF THE ACID IN THE BATH; AND THE BALANCE WATER. 